A projection lens having the smallest possible wavefront aberrations is often desirable to ensure that the mask structures are imaged onto the wafer as precisely as possible. Projection lenses are therefore equipped with manipulators which make it possible to correct wavefront aberrations via a change in state of individual optical elements of the projection lens. Examples of such a change in state include: a change of position in one or more of the six rigid-body degrees of freedom of the relevant optical element, application of heat and/or cold to the optical element, and a deformation of the optical element.
To correct aberrations of the projection lens occurring over the course of time, the travels of the manipulators are regularly readjusted. For this purpose, a travel establishing device establishes a travel command which contains travel settings for the individual manipulators of the projection lens. The readjustment of the manipulator travels is generally effected in different adjustment stages. A first of the adjustment stages includes a maintenance adjustment performed at relatively long intervals, such as of approximately a few weeks or approximately one month. Another of the adjustment stages includes an operating adjustment performed during exposure operation of the projection exposure apparatus with a high cycle rate of at least one repetition per second, in particular of at least 1000 repetitions per second.
During the operating adjustment, the aberration characteristic of the projection lens is usually measured regularly and, if appropriate, changes in the aberration characteristic between the individual measurements are determined by simulation. In this regard, for example, lens element heating effects can be taken into account computationally. In this context, a “lens element heating” is understood to mean not only the heating of a transmission lens element but also the heating of a mirror. The manipulator changes to be performed for correcting the aberration characteristic are calculated via a travel generating optimization algorithm, which is also designated as “manipulator change model” or “lens element model”. Such optimization algorithms are described for example in WO 2010/034674 A1.
The angular distribution of the exposure radiation that is present upon entering the projection lens is influenced by the so-called illumination mode used during the exposure process and also the structure type of the mask structures imaged in this case. Illumination mode and structure type of the imaged mask structures are application-specific and are often designated as “UseCase”. Information regarding illumination mode and/or structure type is designated below as application-specific structure information.
Taking account of the application-specific structure information for calculating the travel command in the context of the operating adjustment conventionally involves the use of time-consuming “fundamental” optimization algorithms for establishing the travel command, which has the effect that the cycle rate achievable in this case is regarded as insufficient for the operating adjustment. Therefore, “fast” optimization algorithms, such as, for instance, optimization algorithms based on Tikhonov regularization, are often used. Such “fast” optimization algorithms, compared with the “fundamental” algorithms, at the expense of accuracy, are often simplified or based on simplified assumptions, such that they can proceed in a shorter time. In the case of the “fast” optimization algorithms used here, it is conventional practice to dispense with taking account of the application-specific structure information on account of the time losses associated therewith. In this regard, in the case of a Tikhonov-regularized optimization algorithm, for instance, taking account of the illumination mode involves subsequent optimization of weighting parameters explained in greater detail below, which in light of the prior art is not automatable, but rather involves a qualified engineer. Therefore, an adaptation of the optimization result to the illumination mode is usually dispensed with in the prior art.
In the case of the maintenance adjustment performed at relatively long intervals, a so-called standard setup is carried out, which is very complex and generally takes up several hours. All the manipulators of the projection lens are preferably used in this case. The manipulators generally include so-called semiactive manipulators alongside conventional manipulators, the driveability of which is not subject to any restrictions. Semiactive manipulators can implement only a very limited number of drivings over their lifetime. The semiactive manipulators include e.g. manipulators for decentrization of lens elements and/or mirrors orthogonally with respect to the optical axis of the projection lens. The degrees of freedom assigned to the semiactive manipulators are also designated as partly active manipulator degrees of freedom in the context of this application. The semiactive manipulators have only a limited influence on the possible performance of the lens and primarily serve for extending the range of the manipulator system of the projection lens over the lifetime of the lens.
On account of the high expenditure of time for carrying out a standard setup, it is conventional practice here to optimize the projection lens without taking account of the application-specific structure information defined by the “UseCase”. Rather, the optimization aims to uniformly minimize the wavefront deviations. The blame for dispensing with taking account of the application-specific structure information in the conventional standard setup lies with practice in semiconductor production, according to which in exposure operation the “UseCase” set at the projection exposure apparatus is changed frequently, e.g. within one day and/or within one week. If the application-specific structure information were taken into account in the standard setup, the standard setup would have to be repeated upon each change of the “UseCase”, which would result in each case in a production outage of at least several hours.